Actually if you rush towards newer siege units then you can easily take cities without worrying about this. A good example is when I took two cities in four turns with three cannons and a musketman. I was the most technologically advanced and so I rushed towards chemistry to grab cannons. Korea the technologically inferior wasn't expecting cannons.
This is minecraft. Why would you want to deal with uneven terrain? If you don't like the lay of the land, you punch it until it shapes the f*** up.
Well, I was trying to use The Huns' Battering Ram because I thought it would give me a nice early rush advantage, but by the time that gets to the city it's already destroyed. I even used the old ploy of sending in weaker military units as distractions, but the AI somehow grew some intelligence with this expansion and targets my siege weapons first.
I decided not to crank out new siege units until I had the Armory so I could throw the Cover upgrade on there. That solved the problem.